This study was performed to clarify the prognosis of patients with surgically treated thymic epithelial tumors. The records of 131 patients who underwent surgical treatment during 1985-2005 were retrospectively reviewed. Pathologic review was done according to the WHO classification of tumors of the thymus. Patients characteristics were: 76 male and 55 fimale; average age 53 (range 20-80) years; tumor stage was stage I in 42, stage II in 43, stage III in 23, stage IVa in 15, stage IVb in 1, and thymic carcinoma (squamous cell carcinoma) in 7 based on Masaoka's staging. There were 7 cases of type A, 23 of type AB, 30 of type B1, 27 of type B2, 29 of type B3, and 15 of type C. Surgical procedures performed were 5 partial resections, 5 tumoretomies, 5 thymectomies, 65 extended thymectomies, 4 tumorectomies plus adjunctive resections of surrounding tissue, and 51 extended thymectomies plus tumorectomies plus adjunctive resections of surrounding tissue including the pleura, pericardium, lung, and great vessels. Five-, 10-, and 15-year survival rates by Masaoka stage were 100%, 100%, and 100% in stage I; 100%, 100%, and 87.5% in stage II; 100%, 87.5%, and 87.5% in stage III; 71.1%, 53.3%, and 53.3% in stage IVa; and 42.9%, 42.9%, and 0% in thymic carcinoma. The prognosis of patients with stage IVa and thymic carcinoma was thus significantly poorer compared with that in the other groups. According to the WHO classification, the 5-year survival rate of type A was 100%, and the 5-, 10-, and 15-year survival rates were 100%, 100%, and 100% in type AB; 100%, 100%, and 75.0% in type B1; 92.6%, 86.4%, and 86.4% in type B2; 95.5%, 95.5%, and 81.8% in type B3; and 57.1%, 42.9%, and 0% in type C. The survival rate of patients with type C was the poorest and there was a significant difference between type C and all other groups. The prognosis of patients with thymic epithelial tumors after resection is thought to be determined by histologic classification and clinical invasiveness. In particular, patients with type B3 and type C thymomas should be considered for multidisciplinary treatment.